Implement Checker for SSD files.

[ClemensLinnhoff]

Is your feature request related to a problem? Please describe.
The integration tests are using SSD files to define the system structure, which is simulated by OpenMCx.
First of all, the files need to conform to the SSD schema, otherwise OpenMCx will throw an error.
And secondly it needs to be checked, that the FMI parameters used by OSI (size, base.low, base.hi) are correctly connected. Otherwise this will lead to undefined behavior during the simulation, which can in some edge-cases be hard to detect.
Connecting e.g. a size output to a base.low input can be a careless mistake, when using graphical tools like easy-ssp.

Describe the solution you'd like

1. Run XML schema validator for every SSD file in the integration tests before starting the co-simulation
2. Implement a checker, that ensures, that a size output is always connected to a size input (same for base.low and base.hi).